[X86] SRL non-LSB extracts when folding to truncating broadcasts.

Now that we recognize this, we can support it instead of bailing out.
That is, we can fold:
  (v8i16 (shufflevector
    (v8i16 (bitcast (v4i32 (build_vector X, Y, ...)))),
    <1,1,...,1>))
into:
  (v8i16 (vbroadcast (i16 (trunc (srl Y, 16)))))

llvm-svn: 252362